having recently flown tiger, the car seat will be included int he 15kg allowance, they are very strict, they even charge for the stroller regardless of weight, iwas told $35 each way for this alone!!! we hired everything in QLD, cot, stroller, carseat as it was cheaper.
have read however that as a car seat is a safety item they should fly them for free, but i doubt tiger would honour that!!

ETA i noticed your going to buy a stroller, it is very cheap to hire, around $20 a week for a layback